It is common practice to suspend the tub of a washing machine in the chassis or the housing of the machine by means of elastic extensible devices such as springs and to absorb the vibration energy produced by the rapid rotation (spin-drying) of the drum loaded with laundry in the washing tub.
An example of such a machine is known from French Patent Specification No. 2,516,952. The suspension arm described in this Patent Specification No. 2,516,952 makes it possible to use a helical spring having bent ends which are hooked into eyes formed in suspension members and to promote the dissipation of the heat produced in the damper, the cylindrical casing which rubs against the tube being situated at the outside of the arm, so that its diameter can be as large as possible.
Said suspension arm has the drawback that it provides a constant damping regardless of the amplitude of the movement caused by the vibrations of the tub and regardless of the speed of rotation of the drum arranged in said tub.
As a matter of fact, a washing machine has a certain number of natural frequencies in its range of operating speeds. In particular, it has a first natural frequency in a range between 50 and 100 r.p.m. and a second natural frequency at approximately 200 r.p.m. At speeds below approximately 100 r.p.m. the exciting force is small. Since the damping cannot be too low because this is undesirable when higher critical speeds are exceeded, it is necessary to adjust the damping to the limit of fouling at a speed corresponding to higher natural frequencies.
Therefore a compromise is made, i.e. an intermediate value is selected for the damping force between the minimum and maximum damping force.
Another drawback of the constant damping is that at high speeds a substantial power is dissipated by the dampers. Since the possibilities of dissipating the heat are limited, this may give rise to very high temperatures, leading to a substantial wear, which prohibits the use of non-lubricated dampers. Therefore, it is necessary to provide a damping for the sole purpose of crossing the critical speeds.
The use of a constant damping also gives rise to additional noise associated with the transition frequency of the damping system.